The Study On The Nonlinear Optical Effects Of Graphene In Resonance System

Graphene has important applications in optoelectronic devices such as photoelectric switches,phase modulators,mode-locked lasers,and filters due to its special energy band structure and excellent electrical and optical properties.However,due to the single-layer atomic properties of graphene,it has a transmittance of up to 97.7%in the visible and near-infrared bands,that is,the effect of light and graphene is relatively weak.The nonlinear optical response of matter is related to the higher-order terms of the optical electric field.Therefore,enhancing the interaction between graphene and light field,and thus enhancing the nonlinear optical response of graphene,is an important research topic in the field of nonlinear optics.In this paper,the nonlinear optical response of monolayer graphene under near-field electric field enhancement is studied.The studies are of great significance in the preparation and design of graphene-based non-linear optical devices.The main contents and conclusions of the paper are as follows:1.We numerically study second harmonic generation?SHG?and third harmonic generation?THG?from graphene covered on silicon gratings by using the finite element method.The grating-assisted graphene plasmon is excited by transverse magnetic?TM?polarized.We use the surface conductivity,and the corresponding surface current density of graphene at fundamental,SHG and THG frequency respectively instead of the effective in-plane permittivity to respresent the real graphene.We focus our study at the mid-infrared frequency,especially at the wavelength of 10.6?m,which is the output wavelength of a carbon dioxide?CO₂?laser.We in detail studied the SHG and THG intensities affected by the Fermi level of the graphene,the period of grating,the incident angle and the graphene carrier mobility.The maximum SHG and THG output is enhanced by about 6 and 8 orders of magnitude,respectively,which compared with those from a graphene on a flat silicon.2.By using the finite element method,we numerically study THG from graphene embedded in resonant waveguide gratings?RWGs?.The guide mode by a proper designed structure is excited under a transverse electric?TE?polarization to enhance THG of a mid-IR wavelengh around 2?m from graphene,which is usful in medical and lase radar applications,but not limit in such the wavelength for the THG enhancement.Compared with the graphene on a planar dielectric layer,the maximum THG output in the RWGs is enhanced by 9 orders of magnitude.We particularly studied THG intensity influenced by the Fermi level of graphene which can be changed by external electric field,the position of graphene in the waveguide,and multimode of waveguide.Further,the resonance wavelength can be tuned by changing the parameters of RWGs to realize the enhanced THG process and the other third-order nonlinear effects from graphene in a broad band.Our numerical results have great significance for design and experimental fabrication of graphene-based nonlinear photonic devices.3.We numerically study the nonlinear optical third harmonic generation of sinusoidal curved graphene which is placed on the dielectric substrate.Under transverse magnetic polarization,the third harmonic generation from graphene is dramatically enhanced at fundamental-mode and several higher-order modes of surface plasmon of graphene.The enhancement factor of third harmonic generation is up to10⁶ compared to that from graphene on a planar dielectric.We study in detail third harmonic generation intensity affected by the Fermi level,grating period,amplitude and the angle of incidence of light.The tunable and enhanced third harmonic generation response at multiple wavelengths show that the sinusoidal-shaped graphene grating has potential applications for graphene based broadband nonlinear photonics.